Back-illuminated image sensors having both frontside and a backside photodetectors

ABSTRACT

A back-illuminated image sensor includes a sensor layer of a first conductivity type having a frontside and a backside opposite the frontside. One or more frontside regions of a second conductivity type are formed in at least a portion of the frontside of the sensor layer. A backside region of the second conductivity type is formed in the backside of the sensor layer. A plurality of frontside photodetectors of the first conductivity type is disposed in the sensor layer. A distinct plurality of backside photodetectors of the first conductivity type separate from the plurality of frontside photodetectors are formed in the sensor layer contiguous to portions of the region of the second conductivity type. A voltage terminal is disposed on the frontside of the sensor layer. One or more connecting regions of the second conductivity type are disposed in respective portions of the sensor layer between the voltage terminal and the backside region for electrically connecting the voltage terminal to the backside region.

TECHNICAL FIELD

The present invention relates generally to image sensors for use indigital cameras and other types of image capture devices, and moreparticularly to back-illuminated image sensors. Still more particularly,the present invention relates to back-illuminated image sensors havingfrontside and backside photodetectors.

BACKGROUND

An electronic image sensor captures images using light-sensitivephotodetectors that convert incident light into electrical signals.Image sensors are generally classified as either front-illuminated imagesensors or back-illuminated image sensors. As the image sensor industrymigrates to smaller and smaller pixel designs to increase resolution andreduce costs, the benefits of back-illumination become clearer. Infront-illuminated image sensors, the electrical control lines orconductors are positioned between the photodetectors and thelight-receiving side of the image sensor. The consequence of thispositioning is the electrical conductors block part of the light thatshould be received by the photodetectors, resulting in poor quantumefficiency (QE) performance, especially for small pixels. Forback-illuminated image sensors, the electrical control lines orconductors are positioned opposite the light-receiving side of thesensor and do not reduce QE performance.

Back-illuminated image sensors therefore solve the QE performancechallenge of small pixel designs. But small pixel designs still have twoother performance issues. First, small pixel designs suffer from lowphotodetector (PD) charge capacity. This is because the first ordercharge capacity scales along with the area of the photodetector. Second,the process of fabricating a back-illuminated sensor consists of bondinga device wafer to an interposer wafer and then thinning the devicewafer. This process produces grid distortions. These grid distortionslead to the misalignments of the color filter array, which increases theamount of pixel-to-pixel color crosstalk.

FIGS. 1( a)-(d) illustrates a method for fabricating a back-illuminatedsensor in accordance with the prior art. FIGS. 1( a)-(d) depict astandard Complementary Metal Oxide Semiconductor (CMOS) wafer 100 thatincludes epitaxial layer 102 disposed on substrate 104. Together,epitaxial layer 102 and substrate 104 form device wafer 106.Alternately, manufacturers can use a silicon-on-insulator (SOI) waferbecause the buried insulating layer provides a natural etch stop for theback-thinning of device wafer 106. Regardless of starting material, griddistortion is an issue with the back thinning process.

FIG. 1( b) depicts a finished device wafer 106. Typically, multipleimage sensors 108 are fabricated in epitaxial layer 102. FIG. 1( c)illustrates the positioning of an interposer wafer 112 just beforebonding. A typical interposer wafer consists of a silicon layer 114 andan adhesive layer 116, such as a CMP silicon dioxide layer. Thefabricated device wafer 106 is bonded to the interposer wafer 112, andsubstrate 104 and a portion of epitaxial layer 102 are removed by firstgrinding, then polishing, and finally etching the last ten to hundredmicrons of silicon.

FIG. 1( d) illustrates a finished wafer 118 and an exploded view of aback-illuminated image sensor 108 in accordance with the prior art.Stress accumulates in insulating layer 120 due to the depositionprocess, and due to the conductive interconnects 122. There are alsostresses in the adhesive layers 116, 124. The thinning of device wafer106 reduces the strength of epitaxial layer 102.

FIG. 2 is an exaggerate distortion pattern due to thinning and stressrelaxation of epitaxial layer 102. The dashed line 200 represents theundistorted wafer map of back-illuminated image sensors, while the solidline 202 depicts a final distorted pattern. The distorted pattern 202 isa problem when fabricating color-filter array 126 (see FIG. 1( d)) on aback-thinned image sensor. Almost all lithography equipment measures thealignment mark locations of eight to twelve image sensors 108 on thefinished wafer 118, and then performs a global alignment. With moderninterferometry techniques, global alignment provides better than tennanometers (nm) alignment tolerances over three hundred millimeters(mm). In other words, global alignment is superior to die-by-diealignment. Also, blading-off a photolithography mask and aligning themask on a die-by-die basis slows equipment throughput, therebyincreasing costs. For a back-thinned wafer, the uncertainty of afinished wafer 118 position due to distortion (also known as overlay) istypically fifty nm to two hundred nm. For small pixels, uncertainties offifty to two hundred nm lead to significant color-filter arraymisalignment, resulting in significant color cross talk. Theseuncertainties must be compared with a front-illuminated sensor where theoverlay is typically less than twenty nm.

Referring again to FIG. 1( d), the prior art back-illuminated imagesensor illustrates how grid distortion can result in color crosstalkbetween pixels. The two-sided arrow 128 represents the misalignment ofthe frontside photodetectors 130 a, 130 b, 130 c with respect to thebackside color filter elements 132 a, 132 b, 132 c of a color filterarray (CFA) when the CFA is fabricated using global alignment. With afrontside photodetector configuration, the grid distortion can result inlight 134 leaking into a target photodetector (e.g., photodetector 130b) from an adjacent misaligned filter element (e.g., 132 a).

SUMMARY

A back-illuminated image sensor includes a sensor layer of a firstconductivity type having a frontside and a backside opposite thefrontside. An insulating layer is disposed over the backside of thesensor layer. A circuit layer is electrically connected to the sensorlayer and formed adjacent the frontside of the sensor layer such thatthe sensor layer is positioned between the circuit layer and theinsulating layer. One or more frontside regions of a second conductivitytype are formed in at least a portion of the frontside of the sensorlayer. A backside region of the second conductivity type is formed inthe backside of the sensor layer. A plurality of frontsidephotodetectors of the first conductivity type is disposed in the sensorlayer adjacent to the frontside. A distinct plurality of backsidephotodetectors of the first conductivity type separate from thefrontside photodetectors are formed in the sensor layer adjacent to thebackside and contiguous to the backside region. A first voltage terminalis electrically connected to at least one of the one or more frontsideregions. One or more connecting regions of the second conductivity typeare formed in respective portions of the sensor layer between the firstvoltage terminal and the backside region for electrically connecting thevoltage terminal to the backside region. A voltage applied to the firstvoltage terminal biases the backside region to a respective voltage.

A second voltage terminal can be disposed on the frontside surface andelectrically connected to at least one of the one or more frontsideregions. A voltage applied to the second voltage terminal biases the oneor more frontside regions to a voltage. The voltage applied to thesecond voltage terminal can be different from the voltage applied to thefirst voltage terminal.

Each frontside photodetector can be paired with a respective backsidephotodetector to form photodetector pairs.

One or more touch-up implant regions of the first conductivity type canbe disposed between the frontside and backside photodetectors in eachphotodetector pair.

An overflow drain can be disposed in a portion of the sensor layerbetween each photodetector pair.

ADVANTAGES

The present invention has the advantage of providing an image sensorwith increased photodetector charge capacity and improved colorcrosstalk performance.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are better understood with reference to thefollowing drawings. The elements of the drawings are not necessarily toscale relative to each other.

FIGS. 1( a)-(d) illustrate a simplified process of fabricating aback-illuminated image sensor.

FIG. 2 is an exaggerate distortion pattern due to thinning and stressrelaxation of epitaxial layer 102 shown in FIG. 1;

FIG. 3 is a simplified block diagram of an image capture device in anembodiment in accordance with the invention;

FIG. 4 is a simplified block diagram of image sensor 306 shown in FIG. 3in an embodiment in accordance with the invention;

FIG. 5 is a schematic diagram illustrating a first exemplaryimplementation for pixel 400 shown in FIG. 4;

FIG. 6 is a schematic diagram illustrating a second exemplaryimplementation for pixel 400 shown in FIG. 4;

FIG. 7 illustrates a cross-sectional view of a portion of a firstback-illuminated image sensor having frontside and backsidephotodetectors in an embodiment in accordance with the invention;

FIG. 8 is a plot of electrostatic potential versus distance along lineA-A′ in FIG. 7;

FIG. 9 depicts a cross-sectional view of a portion of a secondback-illuminated image sensor having frontside and backsidephotodetectors in an embodiment in accordance with the invention;

FIG. 10 is a flowchart of a method for fabricating a portion of theimage sensor shown in FIG. 9 in an embodiment in accordance with theinvention;

FIG. 11 illustrates a cross-sectional view of a portion of a thirdback-illuminated image sensor having frontside and backsidephotodetectors in an embodiment in accordance with the invention;

FIG. 12 is a plot of electrostatic potential versus distance along linesB-B′ and C-C′ in FIG. 11;

FIG. 13 depicts a cross-sectional view of a portion of a fourthback-illuminated image sensor having frontside and backsidephotodetectors in an embodiment in accordance with the invention;

FIG. 14 illustrates a cross-sectional view of a portion of a fifthback-illuminated image sensor having frontside and backsidephotodetectors in an embodiment in accordance with the invention; and

FIG. 15 depicts a cross-sectional view of a portion of a sixthback-illuminated image sensor having frontside and backsidephotodetectors in an embodiment in accordance with the invention.

DETAILED DESCRIPTION

Throughout the specification and claims the following terms take themeanings explicitly associated herein, unless the context clearlydictates otherwise. The meaning of “a,” “an,” and “the” includes pluralreference, the meaning of “in” includes “in” and “on.” The term“connected” means either a direct electrical connection between theitems connected or an indirect connection through one or more passive oractive intermediary devices. The term “circuit” means either a singlecomponent or a multiplicity of components, either active or passive,that are connected together to provide a desired function. The term“signal” means at least one current, voltage, or data signal.

Additionally, directional terms such as “on”, “over”, “top”, “bottom”,are used with reference to the orientation of the Figure(s) beingdescribed. Because components of embodiments of the present inventioncan be positioned in a number of different orientations, the directionalterminology is used for purposes of illustration only and is in no waylimiting. When used in conjunction with layers of an image sensor waferor corresponding image sensor, the directional terminology is intendedto be construed broadly, and therefore should not be interpreted topreclude the presence of one or more intervening layers or otherintervening image sensor features or elements. Thus, a given layer thatis described herein as being formed on or formed over another layer maybe separated from the latter layer by one or more additional layers.

And finally, the terms “wafer” and “substrate” are to be understood as asemiconductor-based material including, but not limited to, silicon,silicon-on-insulator (SOI) technology, doped and undoped semiconductors,epitaxial layers formed on a semiconductor substrate, and othersemiconductor structures.

Referring to the drawings, like numbers indicate like parts throughoutthe views.

FIG. 3 is a simplified block diagram of an image capture device in anembodiment in accordance with the invention. Image capture device 300 isimplemented as a digital camera in FIG. 3. Those skilled in the art willrecognize that a digital camera is only one example of an image capturedevice that can utilize an image sensor incorporating the presentinvention. Other types of image capture devices, such as, for example,cell phone cameras, scanners, and digital video camcorders, can be usedwith the present invention.

In digital camera 300, light 302 from a subject scene is input to animaging stage 304. Imaging stage 304 can include conventional elementssuch as a lens, a neutral density filter, an iris and a shutter. Light302 is focused by imaging stage 304 to form an image on image sensor306. Image sensor 306 captures one or more images by converting theincident light into electrical signals. Digital camera 300 furtherincludes processor 308, memory 310, display 312, and one or moreadditional input/output (I/O) elements 314. Although shown as separateelements in the embodiment of FIG. 3, imaging stage 304 may beintegrated with image sensor 306, and possibly one or more additionalelements of digital camera 300, to form a compact camera module.

Processor 308 may be implemented, for example, as a microprocessor, acentral processing unit (CPU), an application-specific integratedcircuit (ASIC), a digital signal processor (DSP), or other processingdevice, or combinations of multiple such devices. Various elements ofimaging stage 304 and image sensor 306 may be controlled by timingsignals or other signals supplied from processor 308.

Memory 310 may be configured as any type of memory, such as, forexample, random access memory (RAM), read-only memory (ROM), Flashmemory, disk-based memory, removable memory, or other types of storageelements, in any combination. A given image captured by image sensor 306may be stored by processor 308 in memory 310 and presented on display312. Display 312 is typically an active matrix color liquid crystaldisplay (LCD), although other types of displays may be used. Theadditional I/O elements 314 may include, for example, various on-screencontrols, buttons or other user interfaces, network interfaces, ormemory card interfaces.

It is to be appreciated that the digital camera shown in FIG. 3 maycomprise additional or alternative elements of a type known to thoseskilled in the art. Elements not specifically shown or described hereinmay be selected from those known in the art. As noted previously, thepresent invention may be implemented in a wide variety of image capturedevices. Also, certain aspects of the embodiments described herein maybe implemented at least in part in the form of software executed by oneor more processing elements of an image capture device. Such softwarecan be implemented in a straightforward manner given the teachingsprovided herein, as will be appreciated by those skilled in the art.

Referring now to FIG. 4, there is shown a simplified block diagram ofimage sensor 306 shown in FIG. 3 in an embodiment in accordance with theinvention. Image sensor 306 typically includes an array of pixels 400that form an imaging area 402. Image sensor 306 further includes columndecoder 404, row decoder 406, digital logic 408, and analog or digitaloutput circuits 410. Image sensor 306 is implemented as aback-illuminated Complementary Metal Oxide Semiconductor (CMOS) imagesensor in an embodiment in accordance with the invention. Thus, columndecoder 404, row decoder 406, digital logic 408, and analog or digitaloutput circuits 410 are implemented as standard CMOS electronic circuitsthat are electrically connected to imaging area 402.

Functionality associated with the sampling and readout of imaging area402 and the processing of corresponding image data may be implemented atleast in part in the form of software that is stored in memory 310 andexecuted by processor 308 (see FIG. 3). Portions of the sampling andreadout circuitry may be arranged external to image sensor 306, orformed integrally with imaging area 402, for example, on a commonintegrated circuit with photodetectors and other elements of the imagingarea. Those skilled in the art will recognize that other peripheralcircuitry configurations or architectures can be implemented in otherembodiments in accordance with the invention.

FIG. 5 is a schematic diagram illustrating a first exemplaryimplementation for pixel 400 shown in FIG. 4. Pixel 400 is a non-sharedpixel that includes photodetector 502, transfer gate 504,charge-to-voltage conversion mechanism 506, reset transistor 508, andamplifier transistor 510, whose source is connected to output line 512.The drains of reset transistor 508 and amplifier transistor 510 aremaintained at potential V_(DD). The source of reset transistor 508 andthe gate of amplifier transistor 510 are connected to charge-to-voltageconversion mechanism 506.

Photodetector 502 is configured as a pinned photodiode,charge-to-voltage conversion mechanism 506 as a floating diffusion, andamplifier transistor 510 as a source follower transistor in anembodiment in accordance with the invention. Pixel 400 can beimplemented with additional or different components in other embodimentsin accordance with the invention. By way of example only, photodetector502 is configured as an unpinned photodetector in another embodiment inaccordance with the invention.

Transfer gate 504 is used to transfer collected photo-generated chargesfrom the photodetector 502 to charge-to-voltage conversion mechanism506. Charge-to-voltage conversion mechanism 506 is used to convert thephoto-generated charge into a voltage signal. Amplifier transistor 510buffers the voltage signal stored in charge-to-voltage conversionmechanism 506 and amplifies and transmits the voltage signal to outputline 512. Reset transistor 508 is used to reset charge-to-voltageconversion mechanism 506 to a known potential prior to readout. Outputline 512 is connected to readout and image processing circuitry (notshown). As shown, the embodiment in FIG. 5 does not include a row selecttransistor when the image is read out using pulsed power supply mode.

Although pixels with floating diffusions can provide added functionalityand better performance, pixels without floating diffusions aresufficient for many applications. FIG. 6 is a schematic diagramillustrating a second exemplary implementation for pixel 400 shown inFIG. 4. Pixel 400 is a three-transistor pixel that includesphotodetector 502, reset transistor 508, amplifier transistor 510, androw select transistor 602. The drains of reset transistor 508 andamplifier transistor 510 are maintained at potential V_(DD). The sourceof reset transistor 508 and the gate of amplifier transistor 510 areconnected to photodetector 502. The drain of row select transistor 602is connected to the source of amplifier transistor 510 and the source ofrow select transistor 602 is connected to output line 512. Photodetector502 is reset directly using reset transistor 508 and the integratedsignal is sampled directly by amplifier transistor 510.

Embodiments in accordance with the invention are not limited to thepixel structures shown in FIGS. 5 and 6. Other pixel configurations canbe used in other embodiments in accordance with the invention. By way ofexample only, a pixel structure that shares one or more componentsbetween multiple pixels can be used in an embodiment in accordance withthe invention.

FIG. 7 illustrates a cross-sectional view of a portion of a firstback-illuminated image sensor having frontside and backsidephotodetectors in an embodiment in accordance with the invention. Someof the elements shown in FIG. 7 are described herein as having specifictypes of conductivity. Other embodiments in accordance with theinvention are not limited to these conductivity types. For example, allof the conductivity types may be reversed in another embodiment inaccordance with the invention.

FIG. 7 depicts portions of three exemplary pixels 700 that can beincluded in image sensor 306. Image sensor 306 includes an activesilicon sensor layer 702 formed with an epitaxial layer having a p-typeconductivity. Sensor layer 702 includes a frontside 704 and a backside706 opposite the frontside 704. An insulating layer 708 is disposed overthe backside 706 and a circuit layer 710 is adjacent the frontside 704,such that the sensor layer 702 is situated between the circuit layer 710and the insulating layer 708. The insulating layer 708 can be fabricatedof silicon dioxide or other suitable dielectric material. The circuitlayer 710 includes conductive interconnects 712, 714, 716, such as gatesand connectors, that form the control circuitry for the image sensor 306and electrically connect the circuit layer 710 to the sensor layer 702.

Each pixel 700 includes a respective frontside photodetector 718 f, 720f, 722 f having a p-type conductivity. Frontside photodetectors 718 f,720 f, 722 f collect charge carriers generated within the sensor layer702 from light 724 incident on the backside 706 of sensor layer 702.

Frontside regions of n-type conductivity 726, 728, 730 are formed in thefrontside of the sensor layer 702. Frontside regions 726, 728, 730 areelectrically connected to a voltage terminal 732 for biasing thefrontside regions 726, 728, 730 to a particular voltage level Vbias. Inthe illustrated embodiment, n-type frontside region 726 is configured asan n-type pinning layer that surrounds and lines the shallow trenchisolation (STI) trench 734, n-type frontside region 728 as an n-typepinning layer that is formed over each photodetector 718 f, 720 f, 722f, and n-type frontside region 730 as a shallow n-well that surrounds ap-type charge-to-voltage conversion mechanism 736. Other n-type regionsthat are included in the embodiment but not shown in FIG. 7 include ashallow n-well that surrounds the p+ nodes of a reset and amplifier(e.g., source follower) transistor. Although not shown in thecross-section of FIG. 7, each of the shallow n-wells 730 surroundingeach charge-to-voltage conversion mechanism 736 are continuouslyconnected together electrically by other n-type implants such as then-type pinning layers 726, 728.

In addition to the frontside photodetectors 718 f, 720 f, 722 f, eachpixel includes a p-type backside photodetector 718 b, 720 b, 722 b. Eachpixel 700 therefore includes a respective frontside and backside p-typephotodetector pair (718 f, 718 b), (720 f, 720 b), (722 f, 722 b) forcollecting photo-generated charge carriers from light 724 incident onbackside 706. FIG. 8 illustrates a plot of electrostatic potentialversus distance along line A-A′ in FIG. 7. Plot 800 depicts theelectrostatic potential when photodetectors 720 f, 720 b are empty(contain zero photo-generated charge carriers). In the embodiment shownin FIG. 7, there are no wells or barriers between the pair ofphotodetectors 720 f, 720 b. Typically, in order to not have wells andbarriers between a photodetector pair, the implant dose of the backsidephotodetector 718 b, 720 b, 722 b is less than the frontsidephotodetector 718 f, 720 f, 722 f. Simulations find that a typicalincrease in photodetector charge capacity for a photodetector pairconfiguration is between twenty-five percent (25%) and seventy-fivepercent (75%) compared to a frontside only photodetector configuration.The increase in photodetector capacity depends on several designfeatures, including, but not limited to, the size of pixels 700 and thethickness of sensor layer 702.

A transfer gate 738 is used to transfer collected photo-generatedcharges from the frontside photodetectors 718 f, 720 f, 722 f and thebackside photodetectors 718 b, 720 b, 722 b to respectivecharge-to-voltage conversion mechanisms 736. Charge-to-voltageconversion mechanisms 736 are configured as p-type floating diffusionsin the illustrated embodiment. Each floating diffusion resides in ashallow n-well 730.

During charge transfer, the voltage on the transfer gate 738 is reducedto zero volts and the electrostatic channel potential underneath thetransfer gate 738 is lower than that of the frontside photodetectors 718f, 720 f, 722 f in an embodiment in accordance with the invention. Inone embodiment in accordance with the invention, the transfer ofphoto-generated charges from the photodetectors 718 f, 718 b, 720 f, 720b, 722 f, 722 b to respective charge-to-voltage conversion mechanisms736 is lag-free when there are no wells or barriers to hinder chargetransfer, the electrostatic potential of the backside photodetectors 718b, 720 b, 722 b is greater than the electrostatic potential of thefrontside photodetectors 718 f, 720 f, 722 f, and the electrostaticpotential of the frontside photodetectors 718 f, 720 f, 722 f is greaterthan the electrostatic channel potential underneath the transfer gate738 during charge transfer.

N-type frontside regions 726, 728 adjacent to the frontside 704 reducedark current due to dangling silicon bonds at the interface betweensensor layer 702 and circuit layer 710. Likewise, n-type backside region740 adjacent to the backside 706 reduces dark current at the interfacebetween sensor layer 702 and insulating layer 708. Like the n-typefrontside regions 726, 728, the n-type backside region 740 can beconnected to voltage terminal 732. In the embodiment shown in FIG. 7,backside region 740 is connected to voltage terminal 732 through n-typeconnecting regions 730, 742, 744.

In another embodiment in accordance with the invention, voltage terminal732 is positioned on insulating layer 708 and electrically connected tobackside region 740. N-type connecting regions 730, 742, 744electrically connect backside region 740 to n-type frontside regions726, 728, 730. A voltage applied to voltage terminal 732 biases bothbackside region 740 and n-type frontside regions 726, 728, 730 to avoltage in an embodiment in accordance with the invention.

Referring now to FIG. 9, there is shown a cross-sectional view of aportion of a second back-illuminated image sensor having frontside andbackside photodetectors in an embodiment in accordance with theinvention. In particular, FIG. 9 illustrates a cross-section of theportions of the three pixels 700 shown in FIG. 7 after the performanceof bonding and thinning procedures (interposer wafer not shown). Aglobal alignment is typically performed using one of several knowntechniques after the sensor layer 702 is thinned. FIG. 10 is a flowchartof a method for fabricating a portion of the image sensor shown in FIG.9 in an embodiment in accordance with the invention. An exemplary globalalignment technique aligns a masking layer to one or more alignmentmarks in a first metal layer 900 using an infrared (IR) aligner (block1000 in FIG. 10). In other embodiments in accordance with the invention,the one or more alignment marks are formed in different layers incircuit layer 710. Additionally, a polysilicon gate layer or a trenchisolation layer can be used to form the first alignment marks.

By way of example only, the masking layer is implemented as aphotoresist layer that masks an etch that defines a pattern or openingsto be formed in a layer. As used herein, the term “aligns”, “aligned”,and “aligning” is defined as registering or substantially registeringthe one or more second alignment marks to the first alignment marks asclosely as possible due to the grid distortion.

The one or more second alignment marks 902 are then etched into theinsulating layer 708 and sensor layer 702 from the backside (block 1002in FIG. 10). Etching the one or more second alignment marks (902 in FIG.9) provides better alignment of the backside photodetector implants andthe CFA in an embodiment in accordance with the invention. In anotherembodiment in accordance with the invention, the one or more secondalignment marks 902 can be formed in the epitaxial layer in the sensorlayer or in a metal layer.

After the second alignment marks 902 are etched, a masking layer isaligned to the second alignment marks and one or more dopants of an nconductivity type are implanted into the backside of sensor layer 702 toform backside region 740. One or more masking layers is then aligned tothe second alignment marks and one or more dopants of the n conductivitytype are implanted to form backside photodetectors 718 b, 720 b, 722 band one or more n-type connecting regions 744 (block 1004 in FIG. 10).The dopants in these implanted areas are then activated with a laseranneal (block 1006 in FIG. 10). A thin spacer layer 904 is optionallydeposited or spun coated on the wafer. An optical component, such asfilter elements 906, 908, 910 of a CFA are then fabricated using the oneor more second alignment marks 902 for alignment (block 1008 in FIG.10). If desired, another spacer layer 912 is optionally deposited orspun coated on the wafer. A microlens array 914, which is anotheroptical component, is then fabricated and aligned to the one or moresecond alignment marks 902 (block 1010 in FIG. 10). Optical componentscan be implemented as diffractive gratings, polarizing elements,birefringent materials, liquid crystals, and light pipes in otherembodiments in accordance with the invention.

One benefit of globally aligning the one or more backside connectingregions 744, backside photodetectors 718 b, 720 b, 722 b, color filterelements 906, 908, 910, and microlens array 914 to the same set ofalignment marks is that any misalignment between these elements is notimpacted by grid distortion.

FIG. 9 will now be used to illustrate how photo-generated chargecarriers are directed to the correct pixel, thereby reducingpixel-to-pixel color crosstalk. By way of example only, assume centerfilter element 908 transmits light propagating in the wavelengthsassociated with the color blue (blue photons). Almost all of the bluephotons generate charge carriers near the surface of backside 706.Charge carrier 916 represents one of these photo-generated chargecarriers. Charge carrier 916 is a hole (h) in the embodiment shown inFIG. 9. If there are no backside photodetectors 718 b, 720 b, 722 b,then charge carrier 916 has a near equal probability of migrating intoeither frontside photodetector 720 f or frontside photodetector 722 f.However, with the photodetector pair configuration shown in FIGS. 7 and9, each backside photodetector 718 b, 720 b, 722 b is aligned with theirrespective filter elements 909, 908, 910. Consequently, charge carrier916 drifts to the center of backside photodetector 720 b and from thereis directed into the correct frontside photodetector 720 f. In summary,aligning the backside photodetectors 718 b, 720 b, 722 b to filterelements 906, 908, 910 reduces pixel-to-pixel crosstalk caused by griddistortions.

Referring now to FIG. 11, there is shown a cross-sectional view of aportion of a third back-illuminated image sensor having frontside andbackside photodetectors in an embodiment in accordance with theinvention. In this embodiment, n-type frontside regions 726, 728, 730are biased at one voltage potential while n-type backside region 740 isbiased at a different voltage potential. The n-type frontside regionsadjacent to frontside 1100 of active silicon sensor layer 1102 arebiased to a known voltage level VbiasA through the voltage terminal 732.N-type backside region 740 is connected to another voltage terminal 1104through n-type connecting regions 1106, 1108, 1110. N-type backsideregion 740 is biased to a known voltage level VbiasB through voltageterminal 1104. In one embodiment in accordance with the invention,voltage terminal 1104 is positioned at the edge of the imaging array(e.g., edge of the array of pixels 400 shown in FIG. 4), and isconnected by one or more contacts from the backside 1112 of sensor layer1102. In one embodiment in accordance with the invention, an additionalground contact is disposed between voltage terminals 732, 1104 toeliminate biasing issues during power-up.

Establishing a voltage difference between frontside 1100 and backside1112 of sensor layer 1102 improves color crosstalk performance bycreating an electric field between the backside 1112 and frontside 1100that forces the photo-generated charge carriers into the nearestphotodetector. This additional electric field allows for the use of athicker sensor layer 1102 with improved color crosstalk performance. Byway of example only, for a 1.4 micrometer by 1.4 micrometer pixel, colorcrosstalk performance typically becomes unacceptable for a sensor layer1102 thickness greater than 2 micrometers. However, for a one voltdifference between backside 1112 and frontside 1100, for a sensor layer1102 thickness of six micrometers, color crosstalk performance is nearlyidentical to the two micrometer thickness. A thicker sensor layer 1102typically has better red and near IR response, which is desirable inmany image sensor applications such as security and automotive.

Each pixel 1114 includes a respective frontside and backside p-typephotodetector pairs (718 f, 718 b), (720 f, 720 b), (722 f, 722 b) forcollecting photo-generated charge carriers from light 724 incident onbackside 1112. Transfer gates 738 are used to transfer collectedphoto-generated charge carriers from the photodetector pairs (718 f, 718b), (720 f, 720 b), (722 f, 722 b) to respective charge-to-voltageconversion mechanisms 736.

Depending on the size of each pixel 1114 and the thickness of sensorlayer 1102, additional touch-up implant regions 1116 of the firstconductivity type (e.g., p conductivity type) can be used to remove anywells and barriers between backside photodetectors 718 b, 720 b, 722 band frontside photodetectors 718 f, 720 f, 722 f. The benefit oftouch-up implant regions 1116 is illustrated in FIG. 12. Solid line 1200shows an exemplary electrostatic potential profile versus distance (forthe zero photo-carriers case) along line B-B′ in FIG. 11 withouttouch-up implant regions 1116. A barrier 1202 is present that preventscharge carriers collected within the backside photodetector region 1204from moving to the frontside photodetector region 1206 and subsequentlyinto the respective charge-to-voltage conversion region. Dashed line1208 shows an exemplary electrostatic potential profile with touch-upimplant regions 1116. The barrier is removed and the photodetector-pairconfiguration now operates lag free.

FIG. 12 illustrates other aspects of a “well engineered” photodetectorpair. The electrostatic potential of the backside 1210 is higher thanthe electrostatic potential for the frontside 1212. Because of thispotential or voltage difference, for some pixel designs the dose of thebackside photodetectors 718 b, 728 b, 722 b may be greater than that ofthe frontside photodetectors 718 f, 720 f, 722 f and still be well andbarrier free. This is rarely the case when the frontside 1212 andbackside 1210 electrostatic potentials are equal. Increasing thephotodetector implant dose increases the photodetector charge capacity.Thus, a “well engineered” photodetector is lag-free (zero wells andbarriers) and maximizes photodetector capacity.

Dashed line 1214 represents an exemplary electrostatic potential profileversus distance (for the zero photo-carriers case) along line C-C′ inFIG. 11. The minimum point 1216 on line 1214 represents the minimumelectrostatic potential between two photodetector pairs, and is commonlyreferred to as the “saddle-point.” Exemplary saddle point locations areidentified as locations 1118 in FIG. 11. Upon illumination, a singlephotodetector pair fills up with photo-generated charge carriers. Atsome point in time the photodetector pair reaches saturation. When theexcess charge spills over the saddle point 1216 (see 1118 in FIG. 11),the excess charge blooms into the adjacent photodetector pair.Pixel-to-pixel blooming can lead to numerous image artifacts including“snowballs”, where one defective photodetector creates a multiple pixeldefect, and “linearity kink”, where the color fidelity at low signallevels is different from that at high signal levels.

Introducing an overflow drain point within between photodetector pairthat is lower in electrostatic potential than the saddle point 1216reduces pixel-to-pixel blooming. In one embodiment in accordance withthe invention, a lateral overflow drain is included within each pixelstructure. In another embodiment in accordance with the invention, anatural overflow drain exists between each photodetector pair (718 f,718 b), (720 f, 720 b), (722 f, 722 b) and their respectivecharge-to-voltage conversion mechanism 736. Typically, this naturaloverflow drain point (e.g., location 1120 in FIG. 11) resides a fewtenths of a micron underneath each transfer gate 738. If the implantdoses in the vicinity of the transfer gates 738 are manipulatedproperly, the natural overflow drain point can be lower than saddlepoint 1216 (1116 in FIG. 11).

If the natural overflow drain (1120 in FIG. 11) is not lower inelectrostatic potential than the pixel-to-pixel saddle point 1216, thena small voltage pulse can be applied to all transfer gates 738 betweenreading out each row of pixels. This small voltage pulse lowers theelectrostatic potential at the natural overflow drain (e.g., 1120 inFIG. 11) and bleeds off the excess charge within the photodetector pairbefore blooming occurs.

Referring now to FIG. 13, there is shown a cross-sectional view of aportion of a fourth back-illuminated image sensor having frontside andbackside photodetectors in an embodiment in accordance with theinvention. The structure depicted in FIG. 13 is similar to that in FIG.12 with the addition of one or more n-type frontside isolation regions1300 and n-type backside isolation regions 1302. The additionalisolation regions 1300, 1302 are formed between neighboringphotodetectors and raise the electrostatic potential of saddle point1216 (FIG. 12) and increase the pixel-to-pixel isolation. Frontsideisolation regions 1300 are implanted during frontside processing andbackside isolation regions 1302 during backside processing in anembodiment in accordance with the invention. In one embodiment inaccordance with the invention, the method depicted in FIG. 10 can beused to form the image sensor shown in FIG. 13 with block 1004 includingthe formation of the one or more backside isolation regions 1302.

FIG. 14 illustrates a cross-sectional view of a portion of a fifthback-illuminated image sensor having frontside and backsidephotodetectors in an embodiment in accordance with the invention. N-typefrontside isolation regions 1400 and p-type channel regions 1402surround frontside photodetectors 718 f, 720 f, 722 f while n-typebackside isolation region 1404 and p-type backside channel region 1406surround backside photodetectors 718 b, 720 b, 722 b in the embodimentshown in FIG. 14. Other embodiments in accordance with the invention mayform n-type isolation regions 1400, 1404 and p-type channel regions1402, 1406 such that the regions partially surround each photodetector.In one embodiment in accordance with the invention, the method depictedin FIG. 10 can be used to fabricate the image sensor shown in FIG. 14with block 1004 including the formation of backside isolation regions1404 or backside channel regions 1406.

N-type frontside and backside isolation regions 1400, 1404 serve severalpurposes. First, like isolation regions 1300, 1302 in FIG. 13, frontsideand backside isolation regions 1400, 1404 improve the isolation betweenphotodetectors. Second, isolation regions 1400, 1404 partially wraparound photodetectors 718 f, 718 b, 720 f, 720 b, 722 f, 722 b,increasing the capacity of the photodetectors. Additionally, p-typefrontside and backside channel regions 1402, 1406 remove wells andbarriers between the backside photodetectors 718 b, 720 b, 722 b andfrontside photodetectors 718 f, 720 f, 722 f. In other embodiments inaccordance with the invention, additional p-type channel regions can beformed between regions 1402, 1406 to reduce or eliminate any residualwells and barriers.

Referring now to FIG. 15, there is shown a cross-sectional view of aportion of a sixth back-illuminated image sensor having frontside andbackside photodetectors in an embodiment in accordance with theinvention. FIG. 15 depicts a cross-sectional view through three n-typemetal oxide semiconductor (NMOS) pixels 1500 with a photodetector pairstructure fabricated using the standard CMOS process (p-epitaxial layerin sensor layer 1502 as starting material). The structure is similar tothe PMOS photodetector pair structure shown in FIG. 7 with the p-typeand n-type implants reversed in conductivity. However, there are severalnotable differences between FIG. 7 and FIG. 15. First, for the NMOSphotodetector pairs (1504 f, 1504 b), (1506 f, 1506 b), (1508 f, 1508b), an n-type channel is created between each photodetector pair withn-type channel regions 1510, but in FIG. 7 the p-type sensor layer 702creates the channel connecting the p-type photodetector pairs. Second,for the NMOS photodetector pairs (1504 f, 1504 b), (1506 f, 1506 b),(1508 f, 1508 b), the p-type sensor layer 1502 is used for isolation andalso for electrically connecting the p-type frontside regions 1512,1514, 1516 to the p-type backside region 1518, but in FIG. 7 the n-typeconnecting regions 742, 744 provide the isolation and electricalconnection.

Otherwise, the exemplary NMOS photodetector pair structure shown in FIG.15 is similar to the exemplary PMOS photodetector pair structure of FIG.7. The p-type frontside regions 1512, 1514, 1516 adjacent the frontside1520 of sensor layer 1502 is connected to a voltage terminal 1522 forbiasing p-type frontside regions 1512, 1514, 1516. The shallow p-typefrontside region 1516 surrounds the n-type charge-to-voltage conversionmechanisms 1524. Transfer gates 1526 control the transfer of charge fromphotodetector pairs (1504 f, 1504 b), (1506 f, 1506 b), (1508 f, 1508 b)to respective charge-to-voltage conversion mechanism 1524. P-typebackside region 1518 is formed in sensor layer. 1502 adjacent to thebackside 1528 and reduces dark current. Insulating layer 1530 issituated adjacent to backside 1528 while circuit layer 1532 is adjacentto frontside 1520. Circuit layer 1532 includes conductive interconnects1534, 1536, 1538, such as gates and connectors that form controlcircuitry for the image sensor 1540.

A portion of the embodiment shown in FIG. 15 can be fabricated using themethod illustrated in FIG. 10. The conductivity type of the one or moredopants used to form backside photodetectors 1504 b, 1506 b, 1508 b isn-type while the conductivity type of the one or more dopants used toform backside region 1518 is p-type. Additionally, the conductivity typeof the one or more dopants used to form one or more channel regions 1510is n-type.

The invention has been described in detail with particular reference tocertain embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention. Additionally, even though specific embodiments of theinvention have been described herein, it should be noted that theapplication is not limited to these embodiments. In particular, anyfeatures described with respect to one embodiment may also be used inother embodiments, where compatible. And the features of the differentembodiments may be exchanged, where compatible.

Parts List

-   100 Standard Complementary Metal Oxide Semiconductor Wafer-   102 epitaxial layer-   104 substrate-   106 device wafer-   108 image sensor-   112 interposer wafer-   114 silicon layer-   116 adhesive layer-   118 finished wafer-   120 insulating layer-   122 conductive interconnects-   124 adhesive layer-   126 color filter array (CFA)-   128 two-sided arrow representing misalignment-   130 a frontside photodetector-   130 b frontside photodetector-   130 c frontside photodetector-   132 a color filter element-   132 b color filter element-   132 c color filter element-   134 light-   200 dashed line representing undistorted wafer map-   202 solid line representing distorted wafer pattern-   300 image capture device-   302 light-   304 imaging stage-   306 image sensor-   308 processor-   310 memory-   312 display-   314 other I/O-   400 pixel-   402 imaging area-   404 column decoder-   406 row decoder-   408 digital logic-   410 analog or digital output circuits-   502 photodetector-   504 transfer gate-   506 charge-to-voltage conversion mechanism-   508 reset transistor-   510 amplifier transistor-   512 output line-   602 row select transistor-   700 pixel-   702 sensor layer-   704 frontside of sensor layer-   706 backside of sensor layer-   708 insulating layer-   710 circuit layer-   712 conductive interconnect-   714 conductive interconnect-   716 conductive interconnect-   718 f frontside photodetector-   718 b backside photodetector-   720 f frontside photodetector-   720 b backside photodetector-   722 f frontside photodetector-   722 b backside photodetector-   724 light-   726 frontside region-   728 frontside region-   730 frontside region-   732 voltage terminal-   734 shallow trench isolation (STI)-   736 charge-to-voltage conversion mechanism-   738 transfer gate-   740 backside region-   742 connecting region-   744 connecting region-   800 plot of electrostatic potential-   900 first metal layer-   902 alignment mark-   904 spacer layer-   906 color filter element-   908 color filter element-   910 color filter element-   912 spacer layer-   914 microlens array-   916 charge carrier-   1000-1010 blocks-   1100 frontside of sensor layer-   1102 sensor layer-   1104 voltage terminal-   1106 connecting region-   1108 connecting region-   1110 connecting region-   1112 backside of sensor layer-   1114 pixel-   1116 touch-up implant regions-   1118 location of saddle-point-   1120 natural overflow drain-   1200 solid line-   1202 barrier-   1204 frontside photodetector region-   1206 backside photodetector region-   1208 dashed line-   1210 electrostatic potential of backside-   1212 electrostatic potential of frontside-   1214 dashed line-   1216 minimum or saddle-point-   1300 isolation region-   1302 isolation region-   1400 frontside isolation region-   1402 frontside channel region-   1404 backside isolation region-   1406 backside channel region-   1500 pixel-   1502 sensor layer-   1504 f frontside photodetector-   1504 b backside photodetector-   1506 f frontside photodetector-   1506 b backside photodetector-   1508 f frontside photodetector-   1508 b backside photodetector-   1510 channel region-   1512 frontside region-   1514 frontside region-   1516 frontside region-   1518 backside region-   1520 frontside of sensor layer-   1522 voltage terminal-   1524 charge-to-voltage conversion mechanism-   1526 transfer gate-   1528 backside of sensor layer-   1530 insulating layer-   1532 circuit layer-   1534 conductive interconnect-   1536 conductive interconnect-   1538 conductive interconnect-   1540 image sensor

1. A back-illuminated image sensor, comprising: a sensor layer of afirst conductivity type having a frontside and a backside opposite thefrontside; an insulating layer disposed on the backside of the sensorlayer; a circuit layer electrically connected to the sensor layer andadjacent to the frontside of the sensor layer; one or more frontsideregions of a second conductivity type formed in at least a portion ofthe frontside of the sensor layer; a backside region of the secondconductivity type formed in the backside of the sensor layer; aplurality of frontside photodetectors of the first conductivity type forconverting light incident on the backside of the sensor layer intophoto-generated charges, wherein the plurality of frontsidephotodetectors is disposed in the sensor layer and adjacent to thefrontside; a discreet plurality of backside photodetectors of the firstconductivity type separate from the plurality of frontsidephotodetectors for converting light incident on the backside of thesensor layer into photo-generated charges, wherein the plurality ofbackside photodetectors is disposed in the sensor layer and contiguousto portions of the backside region of the second conductivity type; afirst voltage terminal disposed on the frontside of the sensor layer;and one or more connecting regions of the second conductivity typedisposed in respective portions of the sensor layer between the firstvoltage terminal and the backside region.
 2. The back-illuminated imagesensor as in claim 1, further comprising a second voltage terminalelectrically connected to at least one frontside region of the secondconductivity type.
 3. The back-illuminated image sensor as in claim 1,wherein each frontside photodetector is paired with a respectivebackside photodetector to form photodetector pairs.
 4. Theback-illuminated image sensor as in claim 3, further comprising anoverflow drain disposed in a portion of the sensor layer between eachphotodetector pair.
 5. The back-illuminated image sensor as in claim 4,further comprising one or more frontside isolation regions of the secondconductivity type disposed in the sensor layer and partially surroundingeach frontside photodetector.
 6. The back-illuminated image sensor as inclaim 5, further comprising one or more backside isolation regions ofthe second conductivity type disposed in the sensor layer and partiallysurrounding each backside photodetector.
 7. The back-illuminated imagesensor as in claim 6, further comprising one or more frontside channelregions of the first conductivity type disposed in the sensor layerbetween two frontside isolation regions and adjacent to each frontsidephotodetector.
 8. The back-illuminated image sensor as in claim 7,further comprising one or more backside channel regions of the firstconductivity type disposed in the sensor layer between two backsideisolation regions and adjacent to each backside photodetector.
 9. Theback-illuminated image sensor as in claim 3, further comprising one ormore touch-up implant regions of the first conductivity type disposedbetween the frontside photodetector and the backside photodetector ineach photodetector pair.
 10. The back-illuminated image sensor as inclaim 1, further comprising one or more frontside isolation regionsdisposed between neighboring frontside photodetectors and one or morebackside isolation regions disposed between neighboring backsidephotodetectors.
 11. An image capture device, comprising: aback-illuminated image sensor, comprising: a sensor layer of a firstconductivity type having a frontside and a backside opposite thefrontside; an insulating layer disposed on the backside of the sensorlayer; a circuit layer electrically connected to the sensor layer andadjacent to the frontside of the sensor layer; one or more frontsideregions of a second conductivity type formed in at least a portion ofthe frontside of the sensor layer; a backside region of the secondconductivity type formed in the backside of the sensor layer; aplurality of frontside photodetectors of the first conductivity type forconverting light incident on the backside of the sensor layer intophoto-generated charges, wherein the plurality of frontsidephotodetectors is disposed in the sensor layer and adjacent to thefrontside; a discreet plurality of backside photodetectors of the firstconductivity type separate from the plurality of frontsidephotodetectors for converting light incident on the backside of thesensor layer into photo-generated charges, wherein the plurality ofbackside photodetectors is disposed in the sensor layer and contiguousto portions of the backside region of the second conductivity type; afirst voltage terminal electrically connected to either the frontside ofthe sensor layer; and one or more connecting regions of the secondconductivity type disposed in respective portions of the sensor layerbetween the first voltage terminal and the backside region forelectrically connecting the first voltage terminal to the backsideregion.
 12. The image capture device as in claim 11, wherein theback-illuminated image sensor further comprises a second voltageterminal electrically connected to at least one frontside region of thesecond conductivity type for biasing the one or more frontside regionsto a voltage.
 13. The image capture device as in claim 11, wherein eachfrontside photodetector is paired with a respective backsidephotodetector to form photodetector pairs.
 14. The image capture deviceas in claim 13, wherein the back-illuminated image sensor furthercomprises an overflow drain disposed in a portion of the sensor layerbetween each photodetector pair.
 15. The image capture device as inclaim 13, wherein the back-illuminated image sensor further comprisesone or more backside isolation regions of the second conductivity typedisposed in the sensor layer and partially surrounding each backsidephotodetector.
 16. The image capture device as in claim 15, wherein theback-illuminated image sensor further comprises one or more frontsidechannel regions of the first conductivity type disposed in the sensorlayer between two frontside isolation regions and adjacent to eachfrontside photodetector.
 17. The image capture device as in claim 16,wherein the back-illuminated image sensor further comprises one or morebackside channel regions of the first conductivity type disposed in thesensor layer between two backside isolation regions and adjacent to eachbackside photodetector.
 18. The image capture device as in claim 17,wherein the back-illuminated image sensor further comprises one or moretouch-up implant regions of the first conductivity type disposed betweenthe frontside photodetector and the backside photodetector in eachphotodetector pair.
 19. The image capture device as in claim 11, whereinthe back-illuminated image sensor further comprises one or morefrontside isolation regions disposed between neighboring frontsidephotodetectors and one or more backside isolation regions disposedbetween neighboring backside photodetectors.